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J. M. CARTWRIGHT RADIO RECEIVING CIRCUIT Filed Nov. 5, 1924 amp Patented Sept. 18, 1928.

UNITED STATES JAMES M. CARTWRIGHT, F CLARKSDALE, MISSISSIPPI.

RADIO RECEIVING CIRCUIT.

Application led November 5. 1924. Serial No. 747,961.

This invention relates to improvements in radio receiving circuits and it consists of certain novel arrangements and combinations herein described and claimed.

fin-,object of the invention is to provide a variable resistance in the ground for the purpose ofovercoming obiectionable radio frequency oscillations originating either in the receiver itself or being imposed upon the 0 receiver by external impulses.

. A further object of the invention is to provide a radio receiving circuit employing a variable resistance in the ground for the purpose of varying' the potential of the grid circuit in a manner clearly brought out below. thereby to more readily accommodate the incoming waves.

Another object of the invention is to provide a radio receivingy circuit employing a plurality of stages of radio frequency amplication, the connections in the circuit of one of these stages being such that it is operated as afrequency changer.

OtherAv objects and advantagesappear in the followingI specification. reference being had to the accompanying drawing Which is a diagram ofva radio receiving circuit incorporating improvements. y

The diagram illustrates a 5-tube circuit. The first four of these tubes. respectively designated RF1. RF. RF3 and RF", constitute four-stages of tuned radio frequency amplificationfthelast tube 5 being employed as a detector. As indicated in one ofthe forego- .35 ing objects of theinvention the third stage RF3 is operated-as a frequency changer vin addition to its amplifying function. because it is made .to control pulsations of both radio and audio frequency.`

Each of the four stages mentioned includes an air-core transformer. These are respectivelv designated 6,7. 8 and. 9. Use is made of a tuning element 10 which may be a transformer of a type identical with 6. 7, etc. Each of the respective transformers. including the tuning transformer 10.. has a variable condenser for the purpose of tuning thereby identifying the circuit as being of the tuned lradio frequency type. These condensers are designated 11, 12, 13, 14 and 15. following the order of the foregoing description.

A lead l-connectsthe antennae with one side'of the primary of the tuning transformer 10. A common negative wire -18 is con- 4nected to-the opposite side of the primary. This wire connects with a binding post 19 which is herein regarded as the negative termmal of the A-battery. The positive terminal of the A-battery is connected to the binding post 20, and to this, in turn, is connected the common positive wire 21. A rheostat 22 furnlshes a control for the A-battery current. It 1s to be noted that the filament of the stages or tubes RF1, RF2 and RF* are connected with the common positive wire 21, the opposlte ends of these filaments being connected with the common negative wire 18. In respect to the stages or tubes RF3 and the detector 5 the negative ends of the filaments, are connected with the common negative wlre 18 but the positive ends are connected with an auxiliary positive wire 23 whlch ultimately communicates with the common positive wire 21 but has an independent rheostat control 24.

This arrangement serves to carry out a particular purpose: The grid returns of stages RF1, RF2 and RF4 are connect/edito the common negativewire 18, in other words the filament minus circuit, by means of wires 25, 26

and 27. But the grid return of the stage' RFsand the detector 5 is connected by means of wires 28 and 29 to the filament plus circuit, 1n other words the auxiliary common positive wlre 23. By virtue of. lconnecting the grid return of stage RF3 to apositive conductor, instead of to a negati-ve conductor as in i stages RF1, RF2 and RF4, the grid ret-urn circuit of Stage RF3 is. opposed. thereby neutralizing the foregoingcircuit of RF1, RF2 and RF* by a decided back pressure. This back pressure can be traced to occur in the A-battery itself.

A vwire 30 connects the ground binding post 31 with. the common negative wire 18. But it is important to observe that a variable resistance 32 is interposed in the wire. The use of such resistance furnishes an important function of the improved receiving circuit because it is by such use that obiectionable radio frequency oscillations can be entirely eliminated from the circuit. It is by means of this resistance that the regenerating effect of the tubes in the radio frequency stages can be nullified.V It also provides a control of the circuit in respect to oscillations originating at external sources. because undesired oscillations can be kept out of the circuit by properly manipulating the variable resistance. The grid circuit of stage RFs has a variable leak'-R and fixed condenser 34. The other radio frequency stages do not employ either CII the variable grid leak nor the fixed condenser. The detector 5 has a fixed grid leak and liondenser 36.

Two distinct B-battery .*oltag'es are imu fl upon the plate circuits of the various tubes. The positive terminal of a B- battery of 671,@ volts is connected to the binding post 37 from which it is distributed in common by a wire 33 to the plates of the stages RF1, RF! and ltF* through branch wires 39. 40' and 4l. The positive terminal of a l5-volt B-battery is connected with the bindingr post 42- from which it is distributed in common by a wire 43 and branches 44 and 45 to the plates of the stage RF3 and the detector 5. Two B-batteries are spoken of, but in practice use can be made of a single battery or an appropriate combination of B-batteries as is not uncommon in practice. The negative B-battery vterminal is connected at'the binding;l post46 which also has a connection. with the negative A binding post 19. A maximum plate voltage of volt B-battery 1s applied at binding post 42 only when hard tubes are used in detector 5 and RF3. Such tubes are reconunendcd.

The operation is as follows: lt has already been pointed out that the `rid returns of stages RF1, RF2 and RF4 are connected in thc filament minus circuit represented by the common negative wire 18, but that the grid return of stage RF3 and ofthe detector are connected to theifilament plus circuit, represented by the auxiliary Wire Q3. The tubes of stages RF1, BF2 andr RF4 operate on 67% plate voltage by virtue of being connected with the wire 38, While the tube of stage RF3 and the detector 5 operate on the lower plate voltage of 45 volts by virtue of being connected with the Wire 43.

, Vhen a radio frequency impulse is impressed on .the circuit through the tuning transformer 10 and is amplified in the radio frequency ytransformers 6 and 7 the condenser 34 begins to charge the saine as in an ordinary detector action. The resistance 33 prevents the accumulation of negative voltage upon the grid of the tube in stage RF3, permitting it to leak ofi1 through tbe filament circuit including the auxiliary wire 23.

As the frequencies of the different Wave lengths vary, the negative potential of the grid 33 also varies with the frequencies. There must obviously then be some method of varying the leakage of the resistance 33 in order to prevent an entire blocking of the grid circuit in stage BF2., but at the same time allowingV the modulated audio frequency Waves to leak off at the proper moment.

It is here that the function of the variable resistance 32 begins. As stated before. this resistance is placed in the ground circuit or lead 30, providinpT a proper path and control for the foregoing` radio frequency carrier waves, of stage RF3. It is thus that the bl cking of the tubes is prevented, eliminating undesired tube oscillations. The same action takes place in the detector tube 5 in con- `junction with the condenser 36, resistance 35 and variable resistance 32.

The tube of stage RF3 is actually doing a double duty by passing Waves at radio frequencies and rectifying; them to audibility. Stage RFJ is passing waves at radio frequencies only. Tube RF3 has an output of Waves at both radio and audio frequencies to stage RF1, which as stated, passes only waves at radio frequencies. The result is the imposition of audio frequency waves upon radio frequency waves constituting a new radio frequency which is passed on to tube-RF4 for amplification at radio frequencies, before being' passed on to detector 5 for rectification to audibility at its output.

The rectification of a radio frequency impulse results, theoretically, in a v smoother modulated curve for the Waves. The effect, then, of theaction of the tube RF* is to smooth out the curve of a given wave and delivering the smooth wave to the tube RIM for amplification. The result is an audio wave of smoother and great-er regularity when delivered at the output of the detector ,5. Another esult is the markeddecrease of atmospheric and local interference.

It is quite commonly known that receivers employing radio frequency amplification through multiple stages are hard to control by virtue of being very unstable. Resistances of various kinds are commonly employed with an attempt at stabilization. This expedient has had varying degrees of success. The circuit herein disclosed seeks to get at the desired result by different methods. The grid return of tube RFS is in the positive A-circuit (wire 23) while the other radio frequency tubes RF1, RFg and RF" have the grid returns in the negative A-circuit (wire i8). The grid return of tube RF".z is thus opposed to the grid return circuits of tubes RF1, BF2 and RF4. This sets up a neutralizing effect by virtue of the electrical back pressure spoken of before.

ln addition to the settingup of this electrical back pressure in the circuit further resistance to the free passage of radio frequency waves is offered by loweringthe plate voltage of tube RF3 from 671/2 to 45 volts. This furnishes a smaller electronic path.

It may be stated that this circuit contemplates the use of aloop aerial, a loop control jack being employed as is quite common in practice.

l claim l. In a radio receiving system including' a plurality of tuned radio frequency amplifying stages arranged in cascade relation. and each stage including,r a three electrode tube. a detector tube coupled for rectifyi'ngthel output of the amplifying; stages, and an adjustable grid leak arranged in the grid-filament llU circuit of a tube previous to the last radio an adjustable grid leak in the grid-filament frequency amplifying stage. circuit of a tube preceding the last amplify- 2. In a radio receiving system including ing stage, and a positive bias connection for 10 a plurality of tuned radio frequency amplifythe grid return circuit of the tube having the ing stages, each stage including a three elecadjustable grid leak.

trode tube, a detector tube coupled for rectifying the output of the amplifying stages, JAMES M. CARTWRIGHT. 

